1. Field of the Invention
The present invention relates to a nominal 3% silicon iron which finds a wide usage as a core material in both power and distribution type transformers and includes a new coating, which when applied to the steel of finish gauge thickness which is thereafter subjected to a heat treatment, will provide the steel with an adherent coating having high interlaminar resistance values, aids in the transformation to the (110) [001] orientation, removes sulfur and carbon from the underlying steel and results in the underlying steel exhibiting improved core loss values at high operating inductions, that is inductions in excess of 15 kilogausses.
2. Description of the Prior Art
Grain oriented silicon steels, that is, those steels having cube-on-edge orientation or that orientation known as (110) [001] in terms of Miller indicies has been in commercial usage in transformers for over 30 years. By reason of the favorable orientation of the grains in a cube-on-edge texture, the silicon steels have developed favorable core loss properties. This orientation is usually accomplished by means of a distinct processing regimen coupled with heat treatments which are effective for producing that favorable cube-on-edge orientation.
More specifically, it has been found that in the case of cube-on-edge orientation, manganese sulfide particles within the microstructure are required in order to permit the favorably oriented grains to grow at the expense of the less favorably oriented grains so that the desired degree of cube-on-edge texture can be obtained within the final product. However, the presence of the manganese sulfide particles themselves are detrimental to the end quality magnetic characteristics exhibited by this steel since they reprecipitate upon cooling to room temperature and thereby adversely affect the magnetic characteristics because they inhibit domain wall movement as a result, coatings were developed which would retard the removal of the sulfur from the steel until the desired degree of orientation has taken place and thereafter the coating was effective in the desulfurizing of the steel during the high temperature treatment.
The coating most popularly employed for effecting this desired result was an MgO coating which reacted with the components on the surface of the steel to form a thin adherent layer of a highly insulating glass type constituent which was effective for producing a high degree of interlaminar resistance in addition to performing its other functions. It has been found, however, that the presence of this coating also inhibits any further decarburization that may take place during such high temperature heat treatment with the result that optimum properties could not be attained in the final heat treated steel.
The coating of the present invention overcomes these difficulties and enhances the degree of grain transformation to attain improved magnetic characteristic, is effective for desulfurizing the steel and in addition, decarburizes the steel still further with the result that improved core loss characteristics are observed in the underlying steel when operated at an induction in excess of 15 kilogausses. As a result, the coating of the present invention has resulted in a new magnetic steel product having improved magnetic characteristics which were not heretofore attainable by the prior art practices employed in commercially produced type M-4 silicon steel.